There are known measuring methods and measuring devices for monitoring electrical systems, such as residual current measuring devices (RCMs) or residual current protective devices (RCDs). These known methods and devices are mainly based on the principle of the galvanically isolated residual current measurement, in which all active conductors of an electric line to be monitored are guided as a primary winding through a measuring current transformer having a current transformer core made of ferromagnetic material and having at least one secondary winding. In the fault-free operation of the electrical system, the vectorial sum of the currents on all live conductors of the line is zero and thus there is no magnetic field surrounding said line. If a fault current occurs due to an insulation fault in the electrical system, for example, which flows through a body outside of the line or to ground, a residual current is the result. The variable magnetic field of said residual current induces a current on the secondary side which can be evaluated and which may trigger a report (RCM) or activate a switching member (RCD) which disconnects the faulty line.
In the simplest version, due to the induction principle, devices of this kind are only capable of detecting temporal changes of the magnetic flux and thus, in practice, can only detect pure alternating fault currents or alternating residual currents. However, loads connected to the electrical system, such as electrical machines having electronic semiconductor components like diodes or thyristors in rectifiers or frequency converters, can also cause residual currents that do not have a purely sinusoidal but a pulsating temporal progression or form a pure direct (residual) current. To be able to detect these types of residual currents as well, methods for AC/DC-sensitive residual current measurement were developed.
These developments include known methods and devices in which the measuring current transformer is integrated as a oscillation-generating element in a push-pull oscillator, and in which direct current magnetizations are compensated by an oscillator current applied at the secondary side.
Also, a method is known in which a current transformer core having a nonlinear magnetization curve is used as a component sensitive to magnetic fields in an oscillator circuit implemented as a relaxation oscillator circuit (relaxation oscillator). The relaxation oscillator circuit causes the nonlinear magnetization curve to be traveled in an oscillating manner between the positive and negative saturation as a result of a controlled secondary-side current flow, wherein the current transformer core switches between two easily distinguishable magnetic states in the course of the oscillation. A temporally modulated oscillator signal can be derived from the temporal progression of the secondary (oscillator) currents, said oscillator signal switching between a state 1 and a state 2, and wherein the duration of the respective signal portions in state 1 and 2 (times of stay) occur as a function of the residual current. The direct portion proportional to the residual current is determined by means of a suitable demodulation in an evaluation circuit by averaging via analog low-pass filtering, for example.
It proves disadvantageous in the known methods for AC/DC-sensitive residual current measurement that unavoidable production-related tolerances in the components of the oscillator circuit and in the evaluation circuit lead to measuring errors and compromise the reliability of the measurement. In particular, a demodulation by analog low-pass filtering makes high demands to the quality of the used components, which can oftentimes only be met by a paired selection of the components, which raises costs. To achieve precise measurement results, the known methods also require observing tightly specified parameters for the component sensitive to magnetic fields. Moreover, the dynamic range is strictly predefined by the choice of components so that the range of application is often limited.